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I have a 3KW Induction motor with a 16A FLC to run .. I've installed a sub board for it supplied by a type C 40A MCB .. I've used a 16A type C MCB at the sub board (local) and used a DOL switch with a thermal overload relay.. The main 40A type C breaker is tripping on start up .. I'm concerned that it's an in-rush issue and the 3KW motor is wanting more juice but i would have thought the 16A local MCB would have gone 1st.. Any help would be hugely appreciated.. Thanks Derek
 
what is the motor coupled to?
take it you have already done all the usual tests I.R. phase balance etc.?
If so remove coupling and wish really hard that the fault is mechanical.....:)
what make of mcb is it? can you adjust the In setting like most mccb's.
If not consider changing to an MCCB to take advantage of this feature.
The motor itself could have an internal turn to turn winding fault which would cause an inrush of anywhere between 16 to 25 times FLC. if so get your rigging and lifting gear out because you are changing the motor.......:)
you really havent given enough detail in your Op to help much buddy. so i have used a scattergun approach to advising.
good luck and let us no how you get on
 
i assume that @ 16A for 3kW that it's single phase. as the 16A MCB is holding. i'd suspect the 40A MCB to be faulty. easiest , cheapest option is to swap it out for a new one. if the problem persists, it's only cost you £3 to prove or disprove.
 
what is the motor coupled to?


Its a vacuum blower used for a CNC router saw
take it you have already done all the usual tests I.R. phase balance etc.?
I have not done IR testing yet as the equipment is split new .. on monday morning i will but i'm doubtful that its a fault with the equipment
If so remove coupling and wish really hard that the fault is mechanical.....:)
Haha I wish but like i say it's all brand new
what make of mcb is it? can you adjust the In setting like most mccb's.
Its a control gear LEB Type C Triple pole MCB with no adjustment setting (not exactly a great make but the job had to be within a low budget , standard!! )
If not consider changing to an MCCB to take advantage of this feature.
This is a good shout or could i go up to type D?

you really havent given enough detail in your Op to help much buddy. so i have used a scattergun approach to advising.
Thanks mate .. I'm new to the forum and every commercial/industrial job i've been on in the past i've worked for companies like offshore where i'm under guidance and the planning is done by engineers in the office.. I was asked by a neighbour to undertake the job to help him out and initially thought i had enough relevant experience as an electrician to plan and select the correct equipment .. The motors FLC is 16A but the running current is 6A . I am running the 3Phase CNC router of the same 40A supply to the sub board and have used a 10A typeC triple pole MCB for the CNC and a 16A type C triple pole MCB for the Vacuum induction motor.. I thought the Type C would allow for the in-rush .. Thanks again


(I have colour coded your responses as it made for a difficult to read post otherwise..cheers Darkwood.)

 
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Can we firstly confirm this is a single phase motor, its FLC seems too high for a standard 3ph induction motor, although you say you have used a 3ph supply this in itself may be for the control kit and yes you would assume the motor too but not always.

The MCB's have only partial discrimination which will mean on inrush its just a game of chance to which will trip.

What does the installation guide recommend?... you say its a new bit of kit then you should have the relevent info' regarding recommended fusing that is now required in the info pack of new machinery.
 
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Sorry for any confusion here .. Like i say I am new to this forum and not sure how to use it in the best manner.. I thank you all for being patient ..
 
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what is the motor coupled to? Its a vacuum blower used for a CNC

All the equipment is split new so i haven't started testing it yet.. I will drop onto that but doubt its equipment related

what make of mcb is it? can you adjust the In setting like most mccb's. Its a Type C 40A MCB at the main db feeding a 3phase 4 way sub-board and from the sub board to the motor its a 16A Type C MCB . I found it unusual that the 40 went before the 16.. they are both the same make of boards and breakers LEB control gear .. Do you think going up to a Type D is a good approach or MCCB better?

you really havent given enough detail in your Op to help much buddy. Its a 3 KW 3 Phase Induction motor with a Overload protection of 16A (which will be the FLC also) the running current is 6A .. I've used a 5.5KW DOL starter with a 12.5- 18A thermal overload relay.. as soon as the DOL switch is engaged it trips the 40A Breaker at the main db.. I had thought the Type C protection would have allowed for the initial In-Rush hence selecting 16A for local protection.. I am also running a 3 Phase CNC router of that sub board with a triple pole 10A type C breaker ..


A standard 3ph induction motor will tend to have a FLC of about 6amps and not the 16amps you are stating, do you have 'motor plate details' you can post?
 
Can we firstly confirm this is a single phase motor, its FLC seems too high for a standard 3ph induction motor, although you say you have used a 3ph supply this in itself may be for the control kit and yes you would assume the motor too but not always.

The MCB's have only partial discrimination which will mean on inrush its just a game of chance to which will trip.

What does the installation guide recommend?... you say its a new bit of kit then you should have the relevent info' regarding recommended fusing that is now required in the info pack of new machinery.

Aright mate .. The guy i'm doing the job for does not pay the extra £1500 per year for technical support and the equipment was just delivered onto site yesterday and all i was given to do the fixed wiring installation was a basic installation guide which had a small table with the running current @ 6A the overload protection @ 16A . the only other info was that it comes with a 32A CEE form 5 Pin plug and requires a separate starting contactor with overload protection and a switched 32A CEE for socket.. I've not actually seen the equipment yet and am going back on monday to try and solve the issues.. It's not been the most ideal job by a long run.. but it is a 3 Phase .. I was matching my FLC with the overload protection
 
I'm pretty sure that at my age I'm surely sliding into the "don't know what he is talking about old git"

But one of the first things I was taught in my dark days was the correct device for the job .........

Unless there is a bloody good reason why not, I would always try and protect a distribution with a fuse, and not an MCB.
 
Aright mate .. The guy i'm doing the job for does not pay the extra £1500 per year for technical support and the equipment was just delivered onto site yesterday and all i was given to do the fixed wiring installation was a basic installation guide which had a small table with the running current @ 6A the overload protection @ 16A . the only other info was that it comes with a 32A CEE form 5 Pin plug and requires a separate starting contactor with overload protection and a switched 32A CEE for socket.. I've not actually seen the equipment yet and am going back on monday to try and solve the issues.. It's not been the most ideal job by a long run.. but it is a 3 Phase .. I was matching my FLC with the overload protection

If you have not yet seen the equipment then I would reserve judgement until you get to site, the FLC of 16amp for a 3kw 3ph motor is questionable although not unheard of but then it wouldn't be a standard induction motor in my experience, had it been 1ph then yes its plausible, are you sure the table isn't given relevent FLC's for both 3ph and 1ph versions?

Its also possible the 16amp is relative to the OCPD ie your mcb rating not the actual motor O/L setting - this would make more sense tbh, if this is the case then it begs the question of why the 40amp is tripping- I would suggest that a short circuit is more the culprit here as this would give no preference to the 16amp against the 40amp.
Until you see the full set-up its all guess work tbh.

PS - £1500 for technical support, not surprised he doesn't pay, they having a laugh right?
 
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I don't think you have a fault or an overload issue, you say in the OP that you have thermal overload relay if it was an overload issue then that would pop.

Also if it was overload the 16amp would pop first. Unless as was suggested you have a myriad of other things coming off that 40amp MCB that when the router starts up takes it over the top......then you have some issue with the 40amp breaker

It may very well be a dodgy breaker, it as been known that new accesories fail.

Try fitting the fuse, honestly my post about it was a little tongue in cheek, but I stand by that unless there is a reason not to I would always design a distribution circuit with a fuse
 
I don't think you have a fault or an overload issue, you say in the OP that you have thermal overload relay if it was an overload issue then that would pop.

Also if it was overload the 16amp would pop first. Unless as was suggested you have a myriad of other things coming off that 40amp MCB that when the router starts up takes it over the top......then you have some issue with the 40amp breaker

It may very well be a dodgy breaker, it as been known that new accesories fail.

Try fitting the fuse, honestly my post about it was a little tongue in cheek, but I stand by that unless there is a reason not to I would always design a distribution circuit with a fuse



It was miss-information on behalf of the lad i'm doing the job for.. when he contacted me he said it was the breaker in the main DB that was tripping.. when i went up today i found out it was actually the local sub-board 16A MCB that was tripping because of the in-rush current .. I had a spare 32A Type C so i put that on and it fired up fine.. I'm either going to order a 16A type D or a 20A type C .. the Thermal overload is set to 16A so i'm quite happy to go up to 20 on the MCB as i'm dubious about the type D doing the job.. what do you think?
 
What is the EFLI? This will determine what mcb may or may not be fitted with the existing cables.

Earlier in post you said it was supplied with a 32A commando plug fitted and instructions to fit a 32A socket etc and set overloads to 16A, to my mind this suggests they are expecting you to fit a 32A supply.
 
Did you check the motor plate for the FLC ... I'm still scratching my head here about a 3ph 3kw motor will a FLC of 16amp?

Motor Current Charts

Check the 3ph (2nd chart) - you're looking at approx 6.5flc for a 3kw, if it runs at 6 then that sounds about right, the 16amp I would suggest is the mcb rating although incorrectly specified as it should be a 20amp B or C on a standard 3kw 3ph induction motor with a normal starting duty.

If this motor had an FLC of 16amps then it would be in excess of 7.5kw rating - so be careful when choosing the correct motor overload device, you have already been given the wrong info once.
 
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